Mass spectrometer



Dec. 23, 1952 N L ND 2,623,179

MASS SPECTROMETER Filed Nov. 23, 1948 2 SHEETS-SI-IEET 2 Inventor": Joseph G. Neulaind,

Hi S Attorney Patented Dec. 23, 1952 loseph G NeulandtScotia, NJ Y1, assignorto Gen eral Electric Company, a corporation of New,

York

AppIi'cat'ion -NOVemb'er-ZS, 1948,Serial No; SL627 5' Ciaims; (o1. est-41.9)

This inventionrelates to" amass spectrometer,

and in: particular: apparatus for: obtaining a mass spectrum;

Primarily; a:massspectrometeriis aniapparatusa for" analyzing amaterialin: gaseousiorml A' gas to be analyzed is introduced. into-Jantevacuated':

chamber at low pressure and ionized generally by' bombardment with astreamoi electronsawith in an ionization chamber; The ions are acceler s ated and focusedby an electrostatic potential into 'a beam which-is formed of" ionsof hetero-: geneous" mass to-charge ratio"; The beam' -of. heterogeneous 'ionsis projected through a strong magnetic field where-it is "separated-into a plu' rality-of beams; eachof -w-hich is formed of ions ofthe same mass-to-charge ratio; i. e, each beam is homogeneous v with respect tomass-to-charge ratio but the severalbeams are'separated according to mass-to charge ratio. emerge" from "the magnetic field; they are selectively'discha'rged upon' a collector'electrode structure thereby'gen'erating ion currents which". are I proportional to the "respective beam intensities;

Byg'means 'of 'indi'cating "and/or recording appa ratus, the ion'; currents are measured to" deter mine the individual'beam intensities:

In the mass spectrometer the relation ofma'ss tocharge ratio as atfunctionjof the controlvari ablesisa'sfollows':

n: ifl

Where Mr represents the radius of. "the r ion. beam; path-within the magneticfieldjfi the strength of" the -magnetic field} and vi 'the strengthl'of the electrostatic field; From this relation. it'is easily seen that if theyariables-"Hiand V are main tained atgivenv-a-lues; an ionibeamhaving a oertain' mass. -to-.charge ratio will renew a definite" radius. Similarly, other ion'beamshaving'diff er a llfi d-f o v,

n1 asampl'e'hasbeen introduce'dfto the ion iza'jt o chamber in a"manner"-hereinbefore dfi mersed-in the magnetic field; Theheterogeneous;

ions" are projected into i thisportion, .but only; those havirig a" *mass-to -charge lratiorsuch :that: r their paths will conform tdtthe raidiusszof :lthe

After the beams envelope will avoid beingprematurely discharged upon'the envelope- Tofurtherinsure that only one homogeneous ion beam at a time Will be measured, collector electrode structure is arranged inthe path of the beams which pass through the curved portion of the envelope to receive-only, the beam which follows the proper path. Either the electrostatic field-or the magnetic field may be varied to measure selectively andl'suceessivelythe separated beams. In this manner a. convenient and relatively inertiale'ss systemlfor procuring a mass spectrum is realized.

Inxobtaining mass spectra, however, it is quite common to encounter "samples from which beam intensities varying from 1 to 10,'000 are produced, As ha's-been heretofore. mentioned, the beam intensities are generally: measured by discharging the selected beamsv upon the collector electrode structure to generate'ion currents, which may be measured- -by m'eans of indicating or recording apparatus.v It. is easily seen that, if the measuring apparatus possesses a constant range, the result-ingmea'surements will be inadequate even for the m'ostinaccurate analysis. It is possible to' install amanually adjustable range changing.

apparatus, but such 'an expedienttrequires the attentionof anoperatorand causes considerable loss of time Moreover, if recording apparatus isemployed"to secure a permanent record of the mass spectruni cr jni'u'sihg' traces appear upon the chart? Therefore; it'is an object'ofi this 'invention to provide'automat-ic'range chan'ging 'apparatus :for amass spectrometer whereby beam intensities of widely varying magnitudes maybe measured.

Anotherobject of this invention is to provide means for recording automatically amass specfrom the 'following' description taken irY-cdiified;

tion with the "accompanying drawing 'm which Fig lis 'a' partially schematic view of in" espectrometr-apparatus suitably. "embodying namvention; Fig, 2.is afrepresentation"of'aftypicalmass spectrum" useful .inide'scribing the invention; Fig S .isla plan viewioffafpre'fefred' form "of'cole lectoneleetrode structure utilized in a"r1' las 's*spec-" trometer :embodying -ithe invention: Fig; 4 "is a" view in section taken on the line 4-4 of Fig. 3; Fig. 5 is a plan view of one of the electrodes of the collector electrode structure; Fig. 6 is a plan View of an alternative form of collector electrode structure; Fig. 7 is a view in section taken on line of Fig. 6; and Fig. 8 is a wiring diagram of the preferred form of circuits employed to obtain a mass spectrum in accordance with the invention.

The mass spectrometer apparatus illustrated in Fig. 1 comprises a spectrometer tube referred to generally by the numeral I. At the upper end of the tube is a glass envelope 2 which is attached to a metal analyzer tube 3 by means of a metal cup 4 having a temperature coefficient which permits vacuum-tight sealing between the envelope 2 and the tube 3. In a similar manner, a glass envelope 5 is attached to the lower end of tube 3 by means of a metal cup E to provide a vacuum-tight enclosure in conjunction with envelope 2. In order that the enclosure may be evacuated, suitable pumping apparatus may be attached to the arm member I.

The gas sample to be analyzed is admitted to the evacuated enclosure through inlet tube 8, which directs the sample into the ionization chamber 9; Within the ionization chamber are a filament i and an electrode 1 I, commonly called the trap, which is maintained at a positive potential with respect to the filament H) by a source of direct current indicated conventionally by battery l2. The filament I0 is connected to battery 12 through leads l3 and I4 and is thereby energized to emit electrons which are accelerated to trap i I. As the accelerated electrons traverse the ionization chamber between filament I0 and trap ll, they ionize by bombardment the gas molecules within the ionization chamber. The ionization chamber is connected to battery i2 by lead l and by a'lead 16 to a potential divider l! which is energized by a relatively high voltage source of direct current (not shown), and the ions are constrained to move downward out of the ionization chamber through the various ac celerating and focusing electrodes I8 by a difference of potential existing between the ionization chamber and the uppermost of the electrodes 18.

The accelerating and focusing electrodes l8 are connected to various points on the voltage divider i1 and perform the function of accelerating, collimating, and projecting the ion beam into the analyzer tube 3 which is maintained at ground potential. The openings shown in electrodes l8 permit the passage of the ions and give the desired electrostatic field configuration. It will be understood by those skilled in the art that the various potentials on the electrodes it as well as the potentials upon the electrodes within the ionization chamber may be varied in any convenient manner to provide the desired accelerated ion beam. i

The ion beam projected into analyzer tube 3 is composed of ions of heterogeneous mass-tocharge ratio, and in order to provide a means for separating the beam into a plurality of beams, each being homogeneous with respect to mass-tocharge ratio, the analyzer tube 3 is curved in its centerportion on a constant radius through approximately 60. About the curved portion of the analyzer tube are placed pole pieces IQ of an electromagnet 20, which may be variably energized by a source of direct current (not shown) shunted by an adjustable resistor 2|. The electromagnet provides a magnetic field whose lines of force traverse the heterogeneous ion beam to exert a force upon the individual ions, causingthem to follow circular paths according to the equation hereinbefore expressed. When the ions emerge from the magnetic field, they are separated into individual ion beams which are homogeneous with respect to mass-to-charge ratio.

It will be understood that some of the ions which pass into the magnetic field will have a mass-to-charge ratio such thatthe force exerted upon them by the magnetic field will be insufiicient to cause their paths to conform to the shape of the analyzer tube 3, and therefore they will be discharged upon the sides of the analyzer tube before they proceed through its entire length. Those of the ion beams which do conform to the shape of the analyzer tube 3 proceed to collector electrode structure 22. Since it is desired to measure only one of the separated ion beams at a time, collector electrodes 24 within structure 22 have openings which are arranged to admit selectively the beams to electrode 23, as will be more fully described in connection with Figs. 3 and 4. A source of direct current indicated conventionally by battery 25 is connected to electrodes 24 to suppress secondary electrons which may be produced by the ions striking collector electrode 23. The ions which strike or impinge upon the collector electrode 23 produce an ion current which is conducted through lead 23 and resistor 23" to ground. Lead 23 may be brought through the envelope 5 by means 01 a glass-tometal seal 5 arranged to support collector electrode 23. The ion current flowing through resistor 23" produces a voltage which is amplified by main amplifier 26 and transmitted through a range changer 21 to a recorder 28. Since the ion current is proportional to the intensity of the ion beam striking collector electrode 23, the defiection of recorder 28 will also be proportional to the ion beam intensity, and therefore the desired measurement of a given ion beam intensity is obtained. It will be understood that the recorder 28 comprises a preferred form of measuring instrument to be employed, but that any current-responsive instrument may be substituted therefore without departing from the scope of the invention.

As has been heretofore explained, either the strength of the magnetic field traversing the'ion beam paths or the accelerating voltage applied to divider ll may be varied to. produce a mass spectrum. In the preferred embodiment illustrated in Fig. 1, the strength of the magnetic field may be varied by adjusting resistor 2|, while a con-' stant voltage is applied to divider ll. It has been found that maintaining a constant accelerating voltage while varying the strength of the mag netic field will result in constant resolution over a wider spectrum, i. e. the ability of the spec:

trometer to measure selectively adjacent ion beams, will be constant over a greater range of mass-to-charge ratios. As the strengthof the magnetic field is varied, ion beams having dif-j ferent mass-to-charge ratios will successively be r swept across the collector. electrode structurev and will fall through the openings therein to electrode 23 to produce a trace upon the recorder 28. A typical mass spectrum which may belob tained in this manner is shown in Fig. 2, wherein the magnetic flux density is plotted alonglthe abscissa, while the beamintensities are plotted along the ordinate. It is of advantage to employ a recorder to obtain the mass spectrumbecause the graph shown in Fig. 2 will then betraced pu upon the recorder, chart. Moreover, the r ateof;

acaemro chart to .:provide ameans. of identifying the particularzion. beams? within: the. mass .spectrum obtained; Furthermore, the: means for. varyingrthe magnetic rfiel'd may bez arranged to sweep auto.-

matically any= portionlof :theimass spectrum in amannerwvell known to :those skilled. in the :art. Itiwill' be.observed;.that the. mass spectrum'shown in Fig. 2; contains peak beamv intensities which may betconveniently recorded on a; recorder havingla single:sensitivityrange:. However; frequent.- lyfth'e peak intensities differ greatly asthas been heretofore mentioned and, when recordedat a single: range; they" cannot be. accurately determined. Therefore, in' accordance: with. an importantfeatura ofmy invention, a supplemental collector electrode 29 isv prdvidedwithin the .collectorzelectrode structure 22. (Fig. 1).. This supplementalelectrode performs the function of:re:- ceivingeach ion beam beforeitreachescollector electrode 23 and discharging it through a resistor to ground; The ion current-flowing through theresistor 30produces a voltage'which is amplified by: advance amplifier 3| and transmitted to range changer 21. The voltage fed into. range changer 2'! is eifective to select the" proper sensitivity for recorder 28*so thatwhen the ion beam reaches collector electrode 23- and discharges thereupon, the recorder'trace will be made tothe properscale. The details of the range changer circuit which operate in conjunction with recorder 28 will be more fully described in con-v ncction with Fig. 8.

Fora better understanding: of collector electrode structuresuitably embodying the supplemental electrode 29, reference is made to Figs. 3, 4; and 5 in which the electrodes corresponding to those shown in Fig. 1 are similarly identified. It will be observed that the collector electrode structure comprises anouter shield member 32 which may be attached to analyzer tube 3 by means of screws 33; The dowel pins 34 are rigidly. attached to the shield member 32 to provide a L means'fo'r accurately locating the collector electrode structure with respect to the analyzer tube.

3; Plate-like electrodes 24 formed of a conductive material such as Nichrome, are attached to shield member 32 by-meansof bolt members 35tand.insulating'spacers 3'6. Spacers 36 are accu-rately ground from such materials as glass or quartz to provide a" suitably exact configuration. Supportedbetweentwo. of the spacers 3t and two of theelectrodes 24' isthe plate-likesupplemental electrode 29,. As may benoted, each of the electrodesisprovided withtanopening to permit the. passageof ionbeams therethrough to collector electrode 23. In addition,,the supplemental electrode 29 is provided with a wire-like member 31, preferably of. tungsten, traversing the opening therein.

The relative positions of the wire-like member 31 attached to supplemental electrode 29' and the opening in the electrode 24 which immediately follows the supplemental electrode in the direction of ion beam travel must be arranged such that, when the separated ion beamsv are swept.

across thecollectorr electrode structure, a particular ion beam will strike the wire-like member 3'! first and impinge with maximum intensity thereonbeforeit fallsthrough the following one of electrodes 24' to strike. collector electrode 23. The actual spacingsiivill be determined by the widthandthe distance. between adjacent ion beamsz. In.general-,-it has been. foundthatgthe distance between, :the adjacent ;edges of-'the wires like member tland, the opening inthe following. electrode 24 should be less, than the distance be-,- tween adjacent ion beams'and. at least halfgthe- Width of an ion beam.

Although a preferred form-of collector. elec trode'structure' has been shown in Figs. 3, 4and 5; considerable structural simplification may? be attained by structure such asthat-shown in Figs;- dandr'l. insulating'members 39, 49 and-may be attached to analyzer tube 3 by means of dowel pins-.41"

and: screws 42. Collector electrodes. 43-and1 l4 are respectively supported by, insulatingwmem- The electrodes 43: and 44' are: wire-like inv formi and preferably; consist. of a The: electrodes. extend from their respective supports.

bers Stand 43.

conductive material, such as tungsten.

and traverse an opening 38' in electrode 38:, which permits the passageof ion beams there:-

through. The spacings between the electrodes:

shouldqbe arranged similarly. to the manneriin whichthe spacings between electrodes 31'and'1 24 mentioned in connection with Figs. 3; hare? arranged. Although the electrodes 43' and. have been shown as traversing the opening 38' in a plane spaced from and approximately parallel to the portionoi shield :memberr38 which: they may be: located in" containsopening t l, any suitable manner so. long as their respective one and then the other.

barn sweep;

amplifier 26 is connected. It may also be advantageous to place supplemental electrodes on either-side of th electrode.

to measure: the beam. intensity, thereby permitting a reversal of beamisweep withoutrextensive structural modification. In addition; if beam definingelectrodes such as electrodes 24 (Figs.

4) are required, plate-like members having ahead of elece openings therein may be placed trodes 63, 44.

Forsuitable utilization of the ion beam cur-- rents fromthehereinbefore described collector electrode structure, a; circuit such as that-shown in Fig. 8 may be employed, wherein components corresponding to those shown in Fig. 1 have been similarly identified. Collector electrode 235 is connected to the input of main amplifier 26 which. may be a conventional direct current feed-back amplifier having an'input resistor 23"" and an output resistor is in the cathode circuit" lhe'output of amplifier 26-is of the last stage. connected through a bucking battery 45 movable arm 46 ofla single-pole, double-throw switch 4'? having stationary contacts 48 and 49. Stationary contact 48 is connectedto the movable arm 50 of a second single-pole, doublethrow switch 5| having'stationary contacts 52 andli. Stationary'contact 52 is connected'to the movable arm 54 of a third single-pole, double-throw switch 55 having stationary contacts 55 and 57. stationary contact 5% of the last named switch is connected to one end of a resistor 58 forming a' ortion of a resistance dividercircuit 59 which comprises series con nected resistors 58, 6t, 6!, 62. On end of re; sistor' E2 is: connectedto; ground while the other to the An outer shield member 38: supports-v which is: employed.

emails connected to stationary contact 49 and to one end of resistor 6|. The other end of resistor BI is connected to stationary contact 53 and to one end of resistor 89, the other end of resistor 88 being connected to stationary contact 51. Lead 83 interconnects th upper end of resistor 82 and stationary contact 49 with one terminal 84 of the actuating element 65 of recorder 28, the other terminal 65 of the actuating element being connected to ground. Actuating element 65 serves to drive pen member 61 in response to a received signal to produce a trace on chart 68.

Th supplemental electrode 31 is connected to the input of advance amplifier 3| which comprises a conventional feed-back amplifier having an input resistance 38. Connected to the plate 69 of the last stage of amplifier 3| is a battery 10 having taps H, 12, 13. The positive terminal of battery 18 is connected to plate 69. Gaseous discharge devices 14, 15, 16 have cathodes 11, 18, 19 interconnected and maintained at ground potential and grids 80, 8|, 82 respectively connected to taps H, 12, 13 of battery 10. The plate circuits of discharge devices 14, 15, it respectively include magnetic relays 88, 84, 85 which are fed from a common source of plate voltage indicated conventionally by 13+. The relays 83, 84, 85 are respectively effective when energized to actuate the movable arms 45, 58, 54 of switches 41, El, 55; The source of plate voltage is fed through a magnetic relay 88 whose energizing circuit is connected to a stationary contact 81' of a single-pole, double-throw switch member 88. The other stationary contact 89 is connected through current limiting resistor 80 to the hereinbefore mentioned sourc of plate voltage B+. Movable arm 9| of switch member 88 is connected through capacitor 92 to ground and also has mounted thereupon a cam or projection 93 which is adapted to be contacted by a proiection 94 upon pen member 61 of recorder 28.

As the separated ion beams are swept across collector electrode structure 22 (Fig. 1), each beam will strike first the supplemental electrode 31, thereby causing the output of advance amplifier 3| to vary accordingly. If the ion beam striking electrode 31 is relatively small, none of the discharge devices 14, 15, 16 will fire since their grids will not be driven sufficiently positive by the variation in the advance amplifier output. The ion beam then proceeds to fall through the electrodes 24 (Fig. 1) to strike collector electrode 23, thereby producing an ion current which appears as a voltage increment across output resistor 45 of main amplifier 26. This voltage is conducted through movable arm Mi of switch 81 which, in its normal position, is in contact with stationary member 29. Since stationary contact 49 is connected to resistor 62 and the input terminal 64 of recorder 28, the voltage appears across the input of recorder 28 and actuates pen member 6! to record a trace which is proportional to the intensity of the beam striking collector electrode 23.

If, however, the beam striking 31 has a relatively greater intensity, the voltage of tap 1| on battery 18 will become enough positive with respect to ground to cause discharged device 14 to thereby energizing relay 83 and causing movable arm 45 to swing to stationary contact 48. Thereupon when this relatively more intense beam strikes collector electrode 23, both resisters til, 62 will be in series with the output of main amplifier appearing across resistor 45.

Lead 83 interconnecting resistor 62 and the'iriput terminal 64 of recorder 28 remains fixed and therefore only the proportion of the voltage which appears across resistor 62 will be fed into actuating element 65. It is quite evident that by this means the range of recorder 28 may be changed, and a larger signal may be measured without driving pen member 81 off scale. If

more intense beams strike supplemental electrode 31, both discharge devices 14 and 15 will fire, thereby placing resistor in series with resistors 6| and 62. Similarly, if still more intense beams are received, discharge device 16 will fire, placing resistor 58 in series with resistors 60, 6|, 82.

After a particular beam has struck supplemental electrode 31 causing one or more of the discharge devices to fire and has passed on to collector electrode 23 to be measured, it is necessary that the discharge devices be extinguished in order that the correct range may be selected for the following beam. It will be observed that, as pen member 61 returns to approximately its zero position, projection 94 will strike the projection 93 on movable arm 9| of switch member 83. I'his causes capacitor 92, which has been charged by the B+ voltage supply while pen member 61 has been recording the beam intensity, to discharge through relay 8%, thereby removing the B+ from the discharge devices 14, 15, 18 and extinguishing them. In this manner, range changer 21 is prepared for selecting the proper range of recorder 28 for the next beam which traverses the collector electrode structure.

To facilitate the identification of the range which has been selected by range changer 21 in order to measure a particular beam intensity, amplifier 26 may be adjusted at zero input to provide a small constant output. This will cause pen member 61 to assume a new zero position, and the range selected may be identified by the distance below the new zero to which pen member momentarily moves when the range is changed. As the pen member traces the peak intensity of the beam and returns to strike projection 93, those of discharge devices 14, 15, 16 which have been previously fired will be extinguished and the pen member will automatically assume the new zero position again.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous changes may be made by those skilled in the art without actuaily departing from the invention. I therefore aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination: a fixed full scale sensitivity recorder having a movable recording member; a source of electrical impulses for energizing said recorder having an output responsive to an input signal; a voltage divider circuit having a portion connected across the input of said recorder; a plurality of switching means individually operable in response to different voltages to conneot successively greater portions of said divider in series with said output of said source whereby the proportion of the impulses energizing said recorder may be changed; a Second source of electrical impulses having an output variable in response to an input signal and being connected to said switching means to supply voltages for operating said switching means, said voltages supplied by said second source being ineffective to operate said switching means to disconnect said divider portions after said portions have once been connected; means for supplying the same input signal first to said second source of impulses and thence to said first source of impulses whereby the correct proportion of the impulse resulting from said same input signal may be selected to energize said recorder to maintain said movable recording member on scale while recording the impulse resulting from said same input signal; and switching means operable in response to movement of said recording member for disconnecting said switching means before another input signal is supplied to said second and first sources of impulses.

2. A system for recording impulses having a wide range of amplitudes comprising a fixed full scale sensitivity recorder having a movable recording member, a source of electrical impulses for energizing said recorder having an output responsive to an input signal, a voltage divider circuit having a portion connected across the input of said recorder, a plurality of grid-controlled gaseous discharge devices individually operable when conducting to connect successively greater portions of said divider in circuit with said output of said source whereby the proportion of the impulses energizing said recorder may be changed, a second source of electrical impulses having an output variable in response to an input signal and being connected to the grids of said discharge devices to cause each of said discharge devices to conduct at different impulse amplitudes, means for supplying the same input signal to said second and first sources of impulses in respective time sequence whereby the correct proportion of the impulse resulting from said same input signal may be pre-selected to energize said recorder to maintain said movable recording member on scale while recording the impulse resulting from said same input signal, and switching means operable in response to movement of said recording member for rendering said discharge devices non-conducting before another input signal is supplied to said second and first sources of impulses.

3. A system for recording impulses having a wide range of amplitudes comprising a fixed full scale sensitivity recorder having a movable recording member; a source of electrical impulses for energizing said recorder having an output responsive to an input signal; a voltage divider having a portion connected across the input of said recorder; a plurality of grid-controlled gaseous discharge devices having current responsive relays in the plate circuits thereof, said relays being individually operable to connect successively greater portions of said divider in circuit with said output of said source whereby the proportion of the impulses energizing said recorder may be changed; a second source of electrical impulses having an output variable in response to an input signal and being connected to the grids of said discharge devices to cause each of said discharge devices to conduct at different impulse amplitudes; means for supplying the same'input signal to said second and first sources of impulses in respective time sequence whereby the correct proportion of the impulse resulting from said same input signal may be pro-selected to energize said recorder to maintain said movable recording member on scale While recording the impulse resulting from said same input signal; and a relay operable in response to the movement of said recording memher for removing the plate energization from said discharge devices before another input signal is supplied to said second and first sources of impulses.

4. In mass spectrometer apparatus having an analyzer tube for separating a beam of ions of heterogeneous mass to charge ratio into beams of homogeneous mass to charge ratio, the improvement which comprises collector electrode structure attached to said analyzer tube including a first conducting plate-like member having an opening therein; a second conducting plate-like member insulatingly supported substantially parallel to and adjacent said first member, said second member having an opening therein of smaller cross-sectional area than the opening in said first member aligned with a portion of said last-named opening; and a wire-like conducting member attached across the opening in said first member adjacent the portion aligned with the opening in said second member.

5. In mass spectrometer apparatus having an analyzer tube for separating a beam of ions of heterogeneous mass to charge ratio, the improvements which comprises collector electrode structure attached to said analyzer tube including a first conducting plate-like member having an opening therein; a second conducting plate-like member insulatingly supported substantially parallel to and adjacent said first member, said second member having an opening therein of smaller cross-sectional area than the opening in said first member aligned with a portion of said last-named opening; a wire-like conducting member attached across the opening in said first member adjacent the portion aligned with the opening in said second member; and a third conducting plate-like member insulatingly supported adjacent said second member on the side opposite that adjacent which said first member is supported, said third member having a portion of its surface aligned with the opening in said second member.

JOSEPH G. NEULAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,138,928 Klemperer Dec. 6. 1938 2,380,439 Hoskins et al. July 31, 1945 2,396,949 Harries Mar. 19, 1946 2,470,745 Schlesman May 17, 1949 2,476,005 Thomas July 12, 1949 

